1. Field of the Invention
The present invention relates to a method and equalizer adapted notably to serial type modems.
Certain international standardization documents for transmission methods such as the STANAG (Standardization NATO Agreement) describe waveforms, to be used for modems (modulators/demodulators), that are designed to be transmitted on serial-type narrow bandwidth channels (3 kHz in general). The symbols are transmitted sequentially at a generally constant modulation speed of 2400 bauds.
Since the transmission channel used (in the HF range of 3 to 30 MHz) is particularly disturbed and since its transfer function changes relatively quickly, all these waveforms have known signals at regular intervals. These signals serve as references and the transfer function of the is channel is deduced from them. Among the different standardized formats chosen, some relate to high-bit-rate modems, working typically at bit rates of 3200 to 9600 bits/s which are sensitive to channel estimation errors.
To obtain a high bit rate, it is furthermore indispensable to use a complex multiple-state QAM (Quadrature Amplitude Modulation) type modulation, and limit the proportion of reference signals to the greatest possible extent so as to maximize the useful bit rate. In other words, the communication will comprise relatively large-sized data blocks between which small-sized reference signals will be inserted.
2. Description of the Prior Art
FIG. 1 shows an exemplary structure of a signal described in the STANAG 4539 in which 256-symbol data blocks alternate with inserted, known 31-symbol blocks (called probes or references), corresponding to about 11% of the total.
To assess the impulse response h(t) of the channel at the nth data blocks, there is a first probe (n−1) placed before the data block and a second probe (n) placed after the data block, enabling an assessment of the transfer function of the channel through the combined impulse response obtained by the convolution of:    the impulse response of the transmitter, which is fixed,    the impulse response of the channel, which is highly variable,    the impulse response of the receiver, which is fixed, these three elements coming into play to define the signal received at each point in time.
To simplify the description, it will be assumed hereinafter that this set forms the impulse response of the channel.
The DFE (Decision Feedback Equalizer) is commonly used in modems corresponding for example to STANAGs (such as the 4285) where the proportion of reference signals is relatively high and the data blocks are relatively short (for example 32 symbols in the 4285).
Another prior art method uses an algorithm known as the “BDFE” (Block Decision Feedback Equalizer) algorithm. This method amounts to estimating the impulse response of the channel before and after a data block and finding the most likely values of symbols sent (data sent) that will minimize the mean square error between the received signal and its estimation from a local impulse response that is assumed to be known.
This algorithm, shown in a schematic view with reference to FIG. 2, consists notably in executing the following steps:    a) estimating the impulse response of the channel having a length of L symbols, it being known that this impulse response is estimated,    b) at the beginning of the data block n comprising N useful symbols, eliminating the influence of the symbols of the probe (n−1) placed before (L−1 first symbols),    c) from the probe (n) placed after the data block, eliminating the participation of the symbols of the probe that are disturbed by the influence of the last data symbols (L−1 symbols),    d) from the sample thus obtained, whose number is slightly greater than the number of data symbols (namely N+L−1), making the best possible estimation of the value of the N useful symbols most probably sent.
The step b) may consider the impulse response of the channel to be equal to h0(t) in the probe before the data block, namely Probe n−1, and the step c) may consider this response to be equal to h1(t) in the probe Prone n after the data block.
The step d) consists, for example, in assuming that the impulse response of the channel evolves linearly between h0(t) and h1(t) all along the data block.
The method according to the invention consists notably in adapting to the speed of evolution of the channel and thus, at all times, having an optimum level of performance while, at the same time, only negligibly increasing the computation power needed.
The description will make use of certain notations adopted, including the following:    en: complex samples sent, spaced out by a symbol and belonging to one of the constellations mentioned further above (known or unknown)    rn: complex samples received (the values of n shall be explained each time and these samples may possible belong to a probe or to data)    L: length of the impulse response (in symbols) of the channel to be estimated    P: the number of symbols of a probe    N : the number of symbols of a data block    dk0 . . . dp−1: known complex values of the symbols of the probe preceding the n+k ranking data block, it being understood that the current block has the rank n.